1. Field of the Invention
The invention relates in general to elevator systems, and more specifically, to improved terminal slowdown apparatus for elevator systems.
2. Description of the Prior Art
It is necessary in elevator systems to provide a redundant, independent means for detecting an overspeed condition of an elevator car as it approaches a terminal floor. The speed monitoring and limiting means may monitor the floor selector. If the floor selector is not operating in a manner which will produce a normal slowdown, an auxiliary speed pattern is produced for controlling terminal slowdown. The speed monitoring and limiting means may monitor the speed pattern generator as the elevator car approaches a terminal. A terminal slowdown pattern is provided in place of the normal deceleration pattern when a malfunction is detected, to decelerate the car into the terminal floor.
Monitoring the floor selector, or speed pattern generator, however, only picks up a malfunction in the specific device monitored, and an overspeed condition due to malfunction of some other component would not be detected. Thus, a speed monitoring system adjacent to a terminal floor which monitors car speed as a function of car position is highly desirable.
Application Ser. No. 628,448 filed Nov. 3, 1975, which application is assigned to the same assignee as the present application now U.S. Pat. No. 4,085,823, discloses a discrete car speed monitoring system. This discrete monitoring system monitors car speed as a function of car position at a plurality of discrete speed checkpoints in the hoistway. The car speed is compared with two reference speeds at most car position checkpoints. If the car speed exceeds the lower but not the upper reference speed, the system attempts to decelerate the car by employing an auxiliary terminal slowdown velocity pattern. If the car speed exceeds the upper reference speed at any checkpoint, the car is forced to make an emergency stop.
Application Ser. No. 813,560, filed July 7, 1977, which application is assigned to the same assignee as the present application, now U.S. Pat. No. 4,128,141, is directed to an improvement in elevator car speed monitoring systems which monitor car speed as a function of discrete car positions adjacent to a terminal floor. In this application, the velocity signal provided as the elevator car passes a checkpoint is modified by a signal responsive to car acceleration. If the car is decelerating, the acceleration signal reduces the absolute magnitude of the velocity signal. If the car is accelerating, it increases it. Thus, the modified velocity signal includes an anticipation factor which takes into account the rate at which the car's speed is changing.
U.S. Pat. No. 3,779,346, which is assigned to the same assignee as the present application, discloses a terminal slowdown monitoring system which continuously monitors the car speed as a function of car position, as the car approaches each terminal floor. In this arrangement, closely spaced markers mounted in the hoistway adjacent to each terminal cooperate with a sensor disposed on the elevator car to provide a continuous speed error signal which is used in s reference circuit to detect overspeed. U.S. Pat. No. 4,067,416, which is assigned to the same assignee as the present application, discloses monitoring means which may be used to insure that this independent terminal slowdown apparatus is operative at all times.
The terminal slowdown monitoring systems of the continuous type which detect car speed versus position relative to a terminal floor, such as disclosed in the hereinbefore mentioned U.S. Pat. No. 3,779,346, provide very accurate and reliable monitoring of the terminal slowdown function. However, systems of this type have a built-in limitation which may result in exceeding desirable jerk limitations under certain operating conditions. The maximum allowable speed at any given distance from a terminal floor is determined by the distance to go and the preselected terminal slowdown deceleration rate. Thus, the maximum allowable speed at a predetermined distance from the terminal floor depends only on the distance and does not consider the present velocity and acceleration rate of the elevator car. For example, at 10 feet from the terminal floor, the maximum allowable velocity would be 9.5 ft/sec for an elevator that is decelerating, or for an elevator that is accelerating. While the maximum allowable speed may be acceptable for the decelerating elevator car, the accelerating elevator car has a jerk limitation and will travel some additional distance while the acceleration is changing to the full deceleration value. This, in effect, allows an accelerating car to run at speeds higher than desired as it approaches a terminal floor. An accelerating car moving towards a terminal floor in the terminal slowdown zone, may occur, for example, on a short run, such as a one-floor run into the terminal floor. The present invention is directed to new and improved terminal slowdown control of the continuous speed versus car position type, which enables an elevator car accelerating towards a terminal floor in a terminal slowdown zone to be placed on the desired slowdown speed pattern or curve as easily and smoothly as a decelerating car, without the risk of nuisance trips which are usually associated with efforts which attempt to shave the margin between maximum allowable normal slowdown speeds and speeds which will place the elevator car on independent terminal slowdown control.